The Islamic State in Iraq and al-Sham (ISIS) isn't merely expanding its territorial control -- it is rapidly becoming the wealthiest and best-equipped terrorist organization in the world. Even the Islamic Republic of Iran, no stranger to extremists, is sounding alarm bells.

As Iraq's army, trained and equipped by the U.S. at an expense of over $15 billion, proves incapable of halting the Sunni militants, Shiite irregulars in central and southern Iraq are preparing for all-out sectarian war. Baghdad, like Damascus to the west, is an administrative prize ISIS seeks. But even more symbolically important is razing Shiite shrines in Najaf where the first Shiite imam, Ali, was entombed, and Karbala where the third Shiite imam Husayn was martyred fourteen centuries ago. ISIS spokesmen state fervently: "The battle will rage in Baghdad and Karbala, so prepare for it." In response even the usually moderate Grand Ayatollah Ali Sistani urged that it is "the legal and national responsibility of whoever can hold a weapon, to hold it to defend the country, the citizens and the holy sites."

The Obama administration claims it will stand alongside the elected government in Baghdad, which is now pleading for coalition airstrikes to halt ISIS' relentless advance. Yet after losing 4,487 lives and spending over $1 trillion, the U.S. seems reluctant to intervene -- believing the harm can be isolated to Iraq and Syria. Similar reasoning influenced withdrawal from Afghanistan, where the Taliban is not only reclaiming its former nation but increasingly threatening Pakistan.

In the meantime, ISIS is swiftly adding to its coffers -- displacing all others as the best-funded NGO of terror. Even the Taliban's fiscal resources now pale in comparison to ISIS' loot, which exceeds $500 million in currency and gold bullion due to recent looting of the Iraq Central Bank branches in Mosul and elsewhere. Funding for equipment and fighters had initially come from private donors in Gulf nations, particularly Kuwait and Qatar. Then, as it expanded territorial control, ISIS began levying taxes and customs duties much like nations do. Next, it moved into the extraction and sale of oil and gas in places like Syria's Deir al-Zur Province. In their current thrust across Iraq, the group has added the oil town of Baiji to its portfolio.

The 9/11 attacks were financed by al-Qaeda with no more than $500,000. Hamas has vexed Israel for years with $70 million, and Hezbollah has done so with between $200-500 million. ISIS is far less centrally controlled and ideologically disciplined than many other Islamist groups, having cast off al-Qaeda's oversight in February 2014. Nonetheless, cash and fervor ensured that a mere 800 or so ISIS fighters overpowered two divisions -- or 30,000 U.S.-trained Iraqi troops -- in a matter of hours at Mosul.

And success is bringing more than land and cash to ISIS. American and European military equipment -- not just Humvees but antiaircraft missiles and even UH-60 Blackhawk helicopters stationed by the Iraqi forces at Mosul -- are in the hands of ISIS fighters who flew two in a victory parade. That high-tech equipment will certainly be deployed not only in Iraq but in Syria too and, in time, possibly against American troops stationed in the Persian Gulf.

Nouri al-Maliki's Shiite government in Iraq and Bashar al-Assad's Alawite regime in Syria lead ISIS' hit list at the moment. But ISIS and the Shiite Hezbollah are already battling each other not only in Syria but across the border in Lebanon. That Mediterranean nation would be a prize for the Salafis who could then launch an attempt to take over Jordan -- ISIS has already released videos calling upon Jordanians to rise up against the Hashemite monarchy. ISIS is virulently antisemitic as well, claiming via Twitter to be behind attacks on Israeli forces in the Golan Heights and lauding former fighters who have attacked Jewish institutions in Europe.

There is no love lost between the ayatollahs of Iran and ISIS either. With the Islamic Republic denouncing the Sunni fighters as takfiris or apostates and martialing revolutionary guards to prevent those militants' control from spreading to Shiite-dominated southern Iraq and to the Iranian plateau. Even the United Arab Emirates and other Persian Gulf Arab states that have supported ISIS may eventually find their rulers targets for takeover on the grounds that they are insufficiently orthodox. Not surprisingly, Saudi security forces recently dismantled domestic terror clusters linked to the terror group.

As territory, wealth and equipment keep being added to its resource stream, ISIS' capabilities to expand, provision and train its cohorts will grow exponentially. The Sunni-Shiite sectarian war currently raging in both Iraq and Syria will boil even hotter and likely draw major regional adversaries like Iran and Saudi Arabia deeper into support of the battling factions. While governments such as the U.S., Britain, France, German, Russia and China take a wait and see position due to internal socioeconomic challenges and geopolitical rivalries, the danger grows ever more potent around the globe. Militants from the Iraq-Syria arena have been arrested in several European countries -- indeed, roughly 80 percent of Westerners joining Islamist cells do so with ISIS.

For all these reasons, ISIS represents the most serious possibility to date of Osama bin Laden's twisted dream of globalized jihad becoming reality. Inaction is likely to have monumental negative implications not only for the Middle East but for the whole world. ISIS and its Islamist cohorts see no territorial, cultural or religious boundaries as restricting their cause. Consequently, its actions cannot be left unchecked. The time to take down the Islamic State is now.

Jamsheed K. Choksy is chairman of the Department of Central Eurasian Studies in Indiana University's School of Global and International Studies. He also is a member of the US National Council on the Humanities at the National Endowment for the Humanities.

Carol E. B. Choksy is an adjunct lecturer of strategic intelligence in Indiana University's School of Informatics and Computing. She also is CEO of IRAD Strategic Consulting, Inc.